This invention relates to hot melt adhesives which exhibit good cold metal bonding and a long open time. In particular, this invention relates to hot melt butene-1 and ethylene copolymer adhesives.
Adhesive open time which we are referring to is the maximum time at which auto-adhesion (adhesion to itself or to a substrate) can take place for material after it is cooled from the melt to room temperature. Hot melt adhesives which exhibit a long open time (greater than 20 minutes), as well as good cold metal bonding, have long been sought for various applications which require a long open time. For example, expansive surfaces to be coated by adhesives such as table tops to which formica is bonded or assembly line auto parts which are coated with adhesive and later contacted with other parts at some point further down the assembly line are particularly suited to the use of hot melt adhesives which display long open times. Sometimes adhesive coated parts must remain uncontacted for as long as several hours, and thus, require longer open times than are exhibited by and characteristic of other polymers typically used in nonpressure sensitive hot melt adhesives.
Hot melt adhesives can be formulated to be pressure sensitive and have an infinite open time but these adhesives are usually soft, tacky and have limited strength and adhesion. Conventional hot melts such as formulations of poly(ethylene/vinylacetate), polyethylenes, polyamides, or polyesters are rigid, form good strong bonds to certain substrates but have short open times usually less than 1 minute. Moreover, these adhesives usually have problems in adhearing to cold metal substrates which is often required for assembly line production.
Solvent applied contact adhesives can be formulated to give good bond strengths and reasonable open times but they require the use of solvents which can be a toxic, a pollutant and a fire hazard. The polybutylene adhesives are unique in that they require no solvents, have long open times, and show improved adhesion to cold metal substrates.
The poly-1 butene polymers are a unique group of olefinic polymers because they crystallize very slowly. The very slow crystallization rate in contrast to the crystallization rates of other polyolefin crystalline polymers such as EVAs, polyethylenes and polypropylenes, has been found, to be beneficial in formulating hot melt adhesives which have very long open times as well as good adhesion and bonding to cold, heat-sink type substrates--metals such as stainless steel and anodized aluminum for example.
U.S. Pat. No. 3,573,240 describes hot melt adhesive compositions for hard cover book binding. The nature of the book binding process is such that an adhesive which rapidly sets and which has an extremely short open time is desired. Column 4, lines 1 through 7 of '240 disclose that mirror amounts i.e., up to about 5% by weight of alpha olefin comonomers such as ethylene and propylene may be present in the butene-1 polymerization system without any substantial loss of the desirable properties displayed by the resultant essentially homopolymeric system. '240 also states in column 2, lines 61 through 63 that the hot melt adhesive products of '240 display good heat stability and rapid setting speed. Thus, '240 teaches that even though up to about 5% by weight of ethylene may be added to the butene-1 polymerization system, the polymerization system exhibits rapid setting speed (short open time). Thus, '240 is inopposite from the teachings of the present invention--that the addition of small amounts of ethylene to the butene-1 polymer in combination with certain resins of the present invention results in extremely long open time.
Polybutylene polymers are composed of linear chain molecules with the regular and spacially ordered arrangement of ethyl side groups, the pendant groups that result when one butene is polymerized across the 1,2 carbon double bond (along an ethylene chain backbone) (U.S. Pat. No. 3,362,940. When cooled from melt, the ethyl side groups initially align in a tetragonal spatial arrangement, developing a little over one half of the ultimate crystallinity (form II). With time, the tetragonal crystalline phase transforms into a stable hexagonal spatial arrangement with subsequent development of additional crystallinity (form I). This is a very slow process, the transformation being completed in the neat polymer over a period of several days.
Butene-1 can be copolymerized with a variety of alpha-olefins to provide useful copolymers such as those taught in U.S. Pat. No. 3,362,940. Butene-1/ethylene copolymers, with ethylene in the 11-20 mole percent range are of special interest in hot melt adhesives, as the ethylene comonomer produces a lower glass transition temperature (Tg) amorphous phase, reduces further the crystallization rate, and reduces the ultimate level of crystallinity in the polymer. Such are advantages in the development of long open time melt adhesives, as a lower Tg polymer and a higher amorphous phase polymer offers wider formulating latitude in combination with compatible resins, waxes, oils, fillers and additives.